Microelectromechanical systems (MEMS) microphones are commonly used in mobile telephones and other consumer electronic devices, embedded systems and other devices. A MEMS microphone typically includes a conductive micromachined diaphragm that vibrates in response to an acoustic signal. The microphone also includes a conductive plate parallel to, and spaced apart from, the diaphragm. The diaphragm and the conductive plate collectively form a capacitor, and an electrical charge is placed on the capacitor, typically by an associated circuit referred to as a “bias circuit” or “bias generator.” The capacitance of the capacitor varies rapidly as the distance between the diaphragm and the plate varies due to the vibration of the diaphragm caused by the acoustic signal. Typically, the charge on the capacitor remains essentially constant during these vibrations, so the voltage across the capacitor varies as the capacitance varies.
The varying voltage may be used to drive a circuit, such as an amplifier or an analog-to-digital converter, to which the MEMS microphone is connected. Such a circuit may be implemented as an application-specific integrated circuit (ASIC). A MEMS microphone connected to a circuit signal processing circuit is referred to herein as a “MEMS microphone system” or a “MEMS system.” A MEMS microphone die and its corresponding ASIC are often housed in a common integrated circuit package to keep leads between the microphone and the ASIC as short as possible, such as to avoid parasitic capacitances caused by long leads.
The sensitivity of a MEMS microphone depends, at least in part, on the bias voltage applied across the diaphragm and the conductive plate, with a higher voltage yielding a higher sensitivity. However, supply voltages (“rail” voltages) within battery-powered electronic circuits, such as hearing aids, mobile telephones and Bluetooth headsets, are typically insufficient to directly bias MEMS microphones. Therefore, DC-to-DC step-up converters, such as charge pumps, are utilized to generate the required bias voltages. However, DC-to-DC step up converters may be temperature sensitive, which causes the sensitivity of conventional MEMS microphones to depend on temperature.
Furthermore, charge pumps are inefficient, and in general, DC-to-DC step-up converters are significant sources of power drain in these circuits. Their use, therefore, negatively influences battery life. As a point of comparison, a typical hearing aid electret condenser microphone (ECM) draws approximately 50 microamps (μA) and does not require a bias voltage. In contrast, a typical MEMS microphone requires more than 100 μA to power its bias generator. Reducing the amount of power required by a MEMS microphone would, therefore, provide a significant advantage.